The most common form of active matrix display is an active matrix liquid crystal display (AMLCD). AMLCD devices are usually made on large glass substrates that are 0.7 mm thick. Two plates are needed for a cell, so that completed displays are just over 1.4 mm thick. Mobile phone manufacturers, and some laptop computer manufacturers, require thinner and lighter displays, and completed cells can be thinned in an HF (hydrofluoric acid) solution, typically to about 0.8 mm thick. Mobile phone manufacturers ideally want the displays to be even thinner, but it has been found that cells below 0.8 mm thick made by this method are too fragile.
The HF thinning is not attractive because it is a wasteful process that uses hazardous chemicals that are difficult to dispose of safely and economically. There is also some yield loss during the etching process due to pitting of the glass. Many mobile applications want to use the thinnest and lightest glass or plastic based displays possible.
The attractiveness of light, rugged and thin plastic AMLCDs as an alternative has long been recognised. Recently, interest in plastic displays has increased even further, partly due to the increased use of colour AMLCDs in mobile phones and PDAs. There has been much research recently into AMLCDs and organic light emitting diode (OLED) displays on plastic substrates. Despite this interest, there is still a need for a plausible manufacturing route for mass production of plastic displays.
A number of different ways have been reported for the manufacture of thin film transistors (TFTs) or displays on plastic substrates.
One technique is described in WO 05/050754, in which a substrate arrangement is manufactured comprising a rigid carrier substrate and a plastic substrate over the rigid carrier substrate. The rigid carrier substrate is released from the plastic substrate after forming pixel circuits and display cells over the plastic substrate. This enables substantially conventional substrate handling, processing and cell making to be employed.
To release the plastic substrate from a glass carrier a heating method is often used. By heating the glass and the plastic substrate, the plastic substrate and the electronic components formed on the substrate are released from the glass carrier.
There are various methods by which the plastic substrate can be separated from the glass carrier. A release process proposed in WO 05/050754 is a laser lift-off process. Laser light at ultraviolet wavelengths is used to cause the lift-off of the plastic substrate from the underlying carrier. It has been suggested that the release process is a photoablation process due to multiple-photon processes, including localised heating. A suggested material for this process is polyimide, which is chosen for its high-temperature stability and high absorption of UV energy.
There are potential problems in using a heating effect to lift-off the plastic substrate from the glass. Sufficient energy is needed to enable lift off to occur, but without damaging either the plastic substrate or the components formed on it, which may result from thermal expansion effects.
When using a laser lift-off process, higher wavelengths within the UV spectrum are preferable because lower wavelengths are absorbed more by the glass substrate, making the laser release less effective. For example commercially available lasers which operate at 308 nm or 351 nm are preferred.
At these higher wavelengths, the energy absorbed in the plastic layer is statistically distributed without complete thermalisation in the plastic polymer molecules. This gives rise to localised heating effects, which can in turn result in damage to the plastic substrate or the components mounted on it. This can also result in partial or poor lift-off from the carrier.
The EPLaR (Electronics on Plastic by Laser Release) process can in principle be used with a variety of different materials, but the ideal substrate for the EPLaR process is considered to be polyimide, but this is not suitable for use with most LCD display effects. An example of EPLaR process uses the following steps:
Begin by spin coating polyimide onto a glass substrate. It is important that the correct polyimide, with low coefficient of thermal expansion, is used. Typically 10·m of polyimide is applied, but layers in the range 3 to 25·m, can be used or more. This polyimide will eventually form the plastic substrate of the flexible display or electronics.
Deposit a silicon nitride passivation layer on the polyimide.
Standard a-Si TFT fabrication.
Laminate an electrophoretic foil onto the TFT array and make interconnects. At this stage, a fully working electrophoretic display is obtained on glass, with a thin polyimide layer between the glass substrate and the TFT array.
Expose the back of the polyimide to a laser that can pass through the glass, but is strongly absorbed in the polyimide. This effectively means the laser must emit in the spectral range 300 to 410 nm. An excimer laser can be used with wavelength 351 nm, but other wavelengths can be used, such as 302 nm. The laser is strongly absorbed within a thin layer of the polyimide (probably a few angstroms), which is ablated. This leaves a very thin layer of polyimide (typically <15 nm) on the glass and releases most of the polyimide layer. The laser released polyimide layer will effectively be the complete 10·m thick.
The polyimides used for the EPLaR process have excellent properties in terms of mechanical strength, maximum process temperature, stability and resistance to process chemicals. The device can be used for reflective and emissive displays, but it has yellow colouration. This is not ideal for transmissive displays, where the light passes through the substrate. Even more critically, the polyimides have random optical birefringence, induced by the spin coating process. This means that the polyimide substrates are not suitable for use with any display effect that uses polarised light, which includes most LCD effects.
Experiments with clear plastic layers (silicone, BCB and parylene) as the substrate and an underlying absorption layer, such as a-Si have been performed by the applicant. These clear plastics were found to have low strength and disintegrate on laser release. In addition, difficulties remain coping with the standard process temperatures of a-Si or LTPS TFTs and probably some of the process chemicals.